Li+ diffusion and its structural basis in the nanocrystalline and amorphous forms of two-dimensionally ion-conducting LixTiS2

The layered fast ion conductor LixTiS2 (x approximate to 2/3) has been prep ared in nanocrystalline (n-LixTiS2) and amorphous (a-LixTiS2) forms. Hence, a direct comparison of the lithium diffusion in a homogeneously and a hete rogeneously disordered material with the-same composition is possible. As a reference system, polycrystalline LixTiS2 (in its hexagonal modification, h-LixTiS2) was also investigated by measuring the temperature and frequency dependencies of the spin-lattice relaxation rate T-1(-1) of Li-7. The acti vation energies for individually jumping ions as obtained from the low-temp erature flanks of the rate maxima are 190, 160, and 70 meV in h-,n-, and a- LixTiS2, respectively. The frequency dependence of T-1 is sublinear for bot h disordered forms, which is explained in terms of the unified site relaxat ion model. Li-7 nuclear magnetic resonance (NMR) spectra of n-LixTiS2 tend, contrary to those of h-LixTiS2 and a-LixTiS2, to decompose into broad and narrow central line components. The relative intensity of the narrow compon ent, which is attributed to fast moving Li ions in the interfacial regions, is temperature dependent and reaches 50% at 360K. A schematic model for th e ionic conduction process in two-dimensional nanocrystalline fast ionic co nductors Is proposed, introducing grain surface pathways as the dominant tr ack for mobile ions.